A typical injection molding machine has a movable platen and a fixed platen. The movable platen is moved toward the fixed platen for closing the mold and away from the fixed platen for opening the mold. Where part configuration and space permits, a plurality of mold levels may be interspersed between the fixed and movable platen. A mold level is basically a core and cavity set, with the core and cavity defining a space therebetween in the mold closed configuration for receiving melt to form an injection molded part. Molds having a plurality of mold levels are referred to as “stack molds” as the core and cavity sets are in effect “stacked” between the fixed and movable platens.
Mold opening and closing requires that the mold levels be synchronized in opening and closing to generally have the core and cavity sets spaced apart by equidistant amounts when the mold is open. In some circumstances, where different parts are being molded in the different levels, other spacings may be desirable. In any case, a device is required to synchronize the mold opening and closing. Such devices are referred to as “centering devices”.
A variety of centering devices exist including lever arm arrangements, rack and pinion arrangements and spline shaft and nut arrangements. The latter are the type at which the present invention is directed. Typical spline shaft and nut arrangements are described in U.S. Pat. No. 6,089,852.
FIG. 1 is a schematic representation of a typical centering device.
A first mold plate 101 would be affixed to a movable platen of an injection molding machine (not illustrated). A fifth mold plate 105 would be mounted to a fixed platen of an injection molding machine (also not illustrated). A third mold plate 103 is mounted between the first mold plate 101 and fifth mold plate 105. Were this a two-level stack mold, this would represent the entire molding assembly. Three and four-level stack molds are however also known. The full arrangement illustrated includes a second mold plate 102 interspersed between the first mold plate 101 and the third mold plate 103. The arrangement further includes a fourth mold plate 104 interspersed between the third mold plate 103 and the fifth mold plate 105.
A spline shaft 107 is secured to the third mold plate 103 by brackets 110 and 114. The spline shaft 107 is journaled in bearings 111 and 112 to permit axial rotation while restraining longitudinal movement relative to the third mold plate 103. The spline shaft would be provided with involute or helical splines which wind in opposite directions (oppositely twisting helices) from a central region of the spline shaft journaled in the bearings 111 and 112 toward opposite ends of the spline shaft. Respective spline nuts 109 and 115 having corresponding teeth for threadedly engaging the spline shaft 107 are secured respectively to the first mold plate 101 and the fifth mold plate 105 by respective brackets 106 and 116. In a four-level mold, relative movement of the second plate 102 and the fourth plate 104 may be controlled with similar but shorter spline shaft and nut arrangements in which a respective second spline shaft is secured to the second level 102 and a third spline shaft is secured to the fourth level 104. The above U.S. patent discusses various arrangements which may be used.
In the arrangement illustrated in FIG. 1, as the first plate 101 is moved away from the fifth plate 105 the spline nut 109 is pulled along the spline shaft 107 causing the shaft to rotate and to pull plate 103 to the left as illustrated as the spline teeth extract themselves from the second spline nut 115. In effect, the arrangement converts linear motion into rotational motion and vice versa to control the mold opening and closing for multi-level molds.
In larger molds with shorter cycle times, significant loads and stress are applied to the centering device. As the spline nuts 109 and 115 run along the involute splines of the spline shaft 107, frictional heating occurs which is transmitted to the spline shaft 107. Over time, particularly in larger applications, the spline shaft 107 may become hot and pose a potential risk to operators.
Another factor to consider is wear of the spline nuts. In general, the spline nuts 109 and 115 would be made of nylon or comparable polymeric material which eventually get eroded by the spline shaft 107 which itself is typically of metal. The frictionally generated heat enhances the rate of wear of the spline nuts 109 and 115.
It is an object of the present invention to provide a spline shaft that is relatively cool to the touch and avoids heating up during mold operation.